Existing microwave antennas include a wide variety of configurations for various applications, such as satellite reception, remote broadcasting, or military communication. The desirable characteristics of low cost, light weight, low profile and mass producibility are provided in general by printed circuit antennas. The simplest forms of printed circuit antennas are microstrip antennas wherein flat conductive elements, such as monopole or dipole antenna elements, are spaced from a single essentially continuous ground plane by a dielectric sheet of uniform thickness. An example of a microstrip antenna is disclosed in U.S. Pat. No. 3,995,277 to Olyphant.
The antennas are designed in an array and may be used for communication systems such as identification of friend/foe (IFF) systems, personal communication service (PCS) systems, satellite communication systems, and aerospace systems, which require such characteristics as low cost, light weight, low profile, and a low sidelobe. The bandwidth and directivity capabilities of such antennas, however, can be limiting for certain applications.
The use of electromagnetically coupled dipole antenna elements can increase bandwidth. Also, the use of an array of dipole antenna elements can improve directivity by providing a predetermined maximum scan angle.
However, utilizing an array of dipole antenna elements presents a dilemma. The maximum grating lobe free scan angle can be increased if the dipole antenna elements are spaced closer together, but a closer spacing can increase undesirable coupling between the elements, thereby degrading performance. This undesirable coupling changes rapidly as the frequency varies, making it difficult to maintain a wide bandwidth.
One approach for compensating the undesirable coupling between dipole antenna elements is disclosed in U.S. Pat. No. 6,417,813 to Durham, which is incorporated herein by reference in its entirety and which is assigned to the current assignee of the present invention. The Durham patent discloses a wideband phased array antenna comprising an array of dipole antenna elements, with each dipole antenna element comprising a medial feed portion and a pair of legs extending outwardly therefrom.
In particular, adjacent legs of adjacent dipole antenna elements include respective spaced apart end portions having predetermined shapes and relative positioning to provide increased capacitive coupling between the adjacent dipole antenna elements. The increased capacitive coupling counters the inherent inductance of the closely spaced dipole antenna elements, in such a manner as the frequency varies so that a wide bandwidth may be maintained.
Each phased array antenna has a desired frequency range, and the ground plane is typically spaced from the array of dipole antenna elements less than about one-half a wavelength of a highest desired frequency. When the frequency is in the GHz range, the separation between the array of dipole antenna elements and the ground plane is less than 0.20 inch at 30 GHz, for example. However, if the frequency of operation of the phased array antenna is in the MHz range, the separation between the array of dipole antenna elements and the ground plane increases to about 19 inches at 300 MHz, for example.
Fielding a phased array antenna with a relatively large separation between the ground plane and the array of dipole antenna elements becomes rather cumbersome because of its bulkiness. Inflatable or collapsible antennas are sometimes used because they are light in weight, easily portable and are easily deployed. For example, U.S. Pat. No. 5,132,699 to Rupp et al. discloses an inflatable phased array antenna formed on one or more generally planar and vertically inclined panels.
Each panel has a continuous outer wall, a continuous inner wall and a plurality of web partitions extending between the inner and outer walls to form a series of inflatable tubular members. The inner wall is covered with a conducting material so that it forms a ground plane, and a plurality of dipole antenna elements are affixed to the web partitions and spaced from the ground plane in a predetermined relationship so that the phased array antenna will operate at the desired frequency range.
As the frequency range decreases from the GHz range to the MHz range, the size of the phased array antenna increases. This presents a problem when a low radar cross section (RCS) mode is required. For example, the RCS of a ship having a phased array antenna operating in the MHz range would be adversely affected because of the increased size of the array.
An ideal antenna is inherently a low RCS structure. An antenna is a transducer whose function is to maximize power transfer from a propagating electromagnetic wave in free space to a receiver. Some antennas, of course, are intended to maximize power transfer from a transmitter to a propagating free space wave, but a basic principle of electromagnetics known as reciprocity insures that the same structure works both ways. Since the ideal antenna maximizes power transfer to the receiver, it minimizes power scattered back to a hypothetical radar, i.e., RCS.
Of course, actual antennas are non-ideal. Typically, the maximum power transfer condition is only satisfied for a limited range of frequencies and for a limited range of incident wave directions. Directional selectivity is often a design goal for antennas, so that the maximum power transfer condition is only met for a small range of directions and power transfer is minimized for undesired directions. However, over its operating range a good antenna is an efficient absorber of incident energy. Therefore, a broadband antenna with broad angular coverage has the potential to be a radar absorptive material (RAM).